A challenge often encountered in the design of optical systems is the controlled diffraction and reflection of optical radiation of selected wavelengths. In the area of integrated optics, reflections are often produced by employing Bragg gratings. For example, in the area of solid state diode lasers, control of the wavelength of the laser is often effected by incorporating etched Bragg reflector gratings on the semiconductor laser substrate (Hunsperger, R. G. (1982), Integrated Optics: Theory and Technology, New York, Springer-Verlag).
Recently, techniques for fabrication of optical waveguides on a silicon substrate have been described. The waveguides may be formed by depositing or growing successive layers of SiO.sub.2, Si.sub.3 N.sub.4 and SiO.sub.2 on top of a Si substrate, while the Bragg reflectors were made in the waveguides by defining a resist pattern with photolithography, then etching a grating through this pattern in to the top SiO.sub.2 cladding layer (C. H. Henry et al., "Compound Bragg Reflection Filters Made by Spatial Frequency Doubling Lithography," J. Lightwave Technology, 7 (9):1379-1385 (1989)). Bragg reflectors are useful in wavelength division demultiplexing of optical communications (R. Adar, et al., "Polarization Independent Narrow Band Bragg Reflection Gratings made with Silica-on-Silicon Waveguides," Apply. Phys. Lett., 60 (15):1779-1781 (1992)), and as external grating used to force diode lasers to emit light in a very narrow band of wavelengths (P. A. Morton et al., "Hybrid Solution Pulse Source using a Silica Waveguide External Cavity and Bragg Reflector," Appl. Phys. Lett., 59(23):2944-2946 (1991)).
Recently, a particularly useful wavelength conversion technique (denoted "balanced phase matching") has been developed, (See Bierlein et al., Appl. Phys. Lett. 56 (18) pp. 1725-1727 (1990) and U.S. Pat. No. 5,028,107), which involves directing the incident optical waves for wavelength conversion through a series of aligned sections of optical materials for wavelength conversion, said sections being selected such that the sum for the series of sections of the product of the length of each section in the direction of alignment and the .DELTA.k, i.e., change in the propagation constant, for that section is equal to about zero, and such that the length of each section is less than its coherence length; wherein either at least one of said materials is optically nonlinear or a layer of nonlinear optical material is provided adjacent to said series during wavelength conversion, or both. This technique is based on the discovery that wavelength conversion can be accomplished by using a series of sections of optical materials wherein the differences in the refractive indices and the section lengths are balanced to control the effects of destructive interference through the series such that the optical waves are phase matched at the end of the series even though they are not phase matched in the individual sections.
Other techniques for wavelength conversion, which are known as "quasi" phase matching techniques, and include periodic domain reversals or internal reflection have also been described (see J. A. Armstrong et al., "Interactions between Light Waves in a Nonlinear Dielectric", Phys. Rev., 127, 1918 (1962)) . Quasi phase matching in optical waveguides has been described using periodically modulated LiNbO.sub.3 which achieve phase matching by periodically reversing the sign of the nonlinear optical coefficient with a period length such that the product of .DELTA.k and period length of the waveguide is about equal to 2N.pi., where N is an odd integer. Periodically domain-inverted channel waveguides utilizing LiNbO.sub.3 are described by J. Webjorn, F. Laurell, and G. Arvidsson in Journal of Lightwave Technology, Vol. 7, No. 10, 1597-1600 (October 1989) and IEEE Photonics Technology Letters, Vol. 1, No. 10, 316-318 (October 1989). Waveguide fabrication is described using titanium to achieve the periodic domain inversion, or using a periodic pattern of silicon oxide on the positive c-face of LiNbO.sub.3 in combination with heat treatment and subsequent proton exchange. G. A. Magel, M. M. Fejer and R. L. Byer, Appl. Phys. Let. 56, 108-110 (1990) disclose LiNbO.sub.3 crystals with periodically alternating ferroelectric domains produced using laser-heated pedestal growth. These structures generated light at wavelengths as short as 407 nm and were relatively resistant to photorefractive damage for structures of this type. However, these periodically modulated waveguides are considered difficult to fabricate and have optical damage thresholds which are too low for many applications.
Recently, a particularly useful wavelength conversion technique based on "quasi" phase matching has been developed, (see U.S. Pat. No. 5,157,754 and van der Poel et al., Appl. Phys. Lett. 57 (20), pp. 2074-2076 (1990)), which involves directing the incident optical waves for wavelength conversion through a single crystal containing a series of aligned sections of optical materials for wavelength conversion selected from (a) materials having the formula K.sub.1-x Rb.sub.x TiOMO.sub.4 where x is from 0 to 1 and M is selected from P and As and (b) materials of said formula wherein the cations of said formula have been partially replaced by at least one of Rb.sup.+, Tl.sup.+ and Cs.sup.+, and at least one of Ba.sup.++, Sr.sup.++ and Ca.sup.++ with the provisos that at least one section is of optical materials selected from (b) and that for optical materials selected from (b) wherein x is greater than 0.8, the cat ions of said formula are partially replaced by at least one of Tl.sup.+ and Cs.sup.+ and at least one of Ba.sup.++, Sr.sup.++, and Ca.sup.++, said sections being selected such that the sum for the series of sections of the product of the length of each section in the direction of alignment and the .DELTA.k for that section is equal to about 2.pi.N where N is an integer other than zero, and such that the nonlinear optical coefficient of at least one section is changed relative to the nonlinear optical coefficient of at least one adjacent section. This technique makes use of the well known advantages of KTiOMO.sub.4 -type materials (where M is P or As), such as high nonlinearity and resistance to damage, as well as quasi phase matching, and provides for changing the sign and/or magnitude of the nonlinear optical coefficient (i.e., "d") to achieve wavelength conversion.
It is well known in the art that incident light for second harmonic generation may be provided using laser diodes. It is also well known that laser diode performance can be affected by optical feedback. See C. E. Wieman et al., "Using Diode Lasers for Atomic Physics", Rev. Sci. Instrum. 62(1) (1991). Optical feedback of some wavelengths can have an undesirable effect on the laser output wavelength, thereby significantly impeding operation of apparatus relying on effective laser operation. On the other hand, optical feedback of appropriate wavelengths can be used to control the center frequency of diode lasers, thereby stabilizing operation of such apparatus. In any case, substantial surface reflection back to a diode laser is generally considered undesirable. U.S. Pat. No. 5,243,676 describes a segmented waveguide suitable for wavelength conversion at a selected wavelength comprising alternating sections of optical materials which are aligned and have refractive indices different from adjacent sections, characterized by a periodic structure along the waveguide which provides a Bragg reflection for said selected wavelength which has a wavelength essentially equal to the wavelength of the input wave. The periodic structure contains at least one superperiod consisting of a plurality of segments (each segment consisting of one section each of two optical materials) wherein at least one segment of the superperiod is different in optical path length from another segment thereof and wherein the sum for the superperiod sections of the product of the length of each section in the direction of alignment and the refractive index of the section is equal to about N.sub.z .lambda./2 where N.sub.z is an integer and .lambda. is the wavelength of the input wave used for wavelength conversion.
Bragg reflectors integrated with proton-exchanged waveguides in LiNBO.sub.3 have also been described. These have been used as selective feedback elements for altering the lasing characteristics of a pump diode laser for frequency doubling (K. Shinozaki et al., "Self-quasi-phase-matched Second-harmonic Generation in the Proton-exchanged LiNbO.sub.3 Optical Waveguide with Periodically Domain-inverted Regions," Appl. Phys. Lett., 59(5):510-512 (1991)), and to form an external resonant cavity containing a frequency-doubling waveguide (K. Shinozaki et al., "Second-harmonic Generation Device with Integrated Periodically Domain-inverted Regions and Distributed Bragg Reflector in a LiNbO.sub.3 Channel Waveguide," Appl. Phys. Lett., 58(18):1934-1936 (1991)).
One of the major difficulties in producing Bragg reflectors photolithographically having the dimensions required by the art structures described above lies in the inexact nature of the photolightgraphic process per se. The Bragg reflectors actually produced may have dimensions different from those that were actually desired due to accidental over exposure or under exposure. In specific terms, consider typical photolithography utilizing a dark-field mask with rectangular openings, a positive-working photoresist on top of a metal layer on top of the substrate, whereby the metal layer is etched through the rectangular openings in the photoresist. In this example, an under exposure through the photomask tends to decrease the size of the openings in the metal mask, whereas over exposure tends to increase the size of the openings. In either case the dimensions of the sections required in the Bragg reflector could be adversely affected.